The invention relates to a linear (e.g., CDMA) RF power amplifier bias networks, and more particularly to a linear RF power amplifier including a dual bias supply for maintaining a constant quiescent operating bias point over temperature and a range of control voltages.
For constant envelope bipolar transistor based power amplifiers (GSM, FM) generally all that is required of a bias network is to set the bias point for the class of operation desired, and to track the base voltage with temperature to maintain a stable operating point. This is easily obtained with a passive bias scheme (current Mirror/constant current CC) as shown in FIG. 1, described below. The “on chip” diode or the diode connected transistor track the RF power device base voltage very well whether implemented in silicon or the lower thermally conductive GaAs substrates. Also with this approach there is little variation in bias point with any change in a control voltage, Vref. This can be important in the typical high volume wireless consumer market where high accuracy/high cost voltage regulators are not required. The main problem with this style of bias supply is its high internal impedance, a resistor 102 when applied to non-constant envelope modulation schemes (e.g., CDMA, W-CDMA, and QPSK). For example, CDMA modulation (IS-95 standard) has a +3 dB to −11 dB peak to average instantaneous (at 1.25 MHz modulation rate) amplitude variations. To support the instantaneous +3 dB (2 times) increase in power the output RF transistor's current will need double (constant supply voltage), requiring the base current to also double. With the basic current mirror (FIG. 1), the resistor 102 is large to keep the bias diode transistor small and to minimize current drain, doubling the supplied current results in doubling the voltage drop across the resistor 102. This drops the base voltage accordingly which is significant as the collector current is exponentially related to the base emitter voltage Vbe. With a lower base voltage the RF power transistor collector current cannot follow the AM (+3 dB) component of the modulation and distortion results. The larger the internal bias resistance, typically 1000 ohms or more, the more distortion occurs.
A more traditional bias approach for non-constant envelope modulation schemes uses a current mirror with a current boost pass transistor (see for example FIG. 2 described below) as a constant voltage source. This approach has the advantage of much lower output resistance, typically 2 orders of magnitude or 100 times less , due to active feedback, thus enabling instantaneous tracking of the linear modulation. This means that the base voltage can be held much more stable under the demands of linear modulation as described above, yielding a more linear RF amplifier especially at higher powers that can result in a higher efficiency linear RF amplifier. Two issues remain for this type of bias; first, the inclusion of the pass transistor reduces the temperature tracking accuracy of the current mirror, and second, the circuit is very sensitive to Vref requiring high accuracy/higher cost voltage regulators.
Another approach, sometimes known as buffered passive bias, includes both resistive and a modified active bias circuit (see for example, FIG. 3 described below). Both parts provide temperature compensation along with minimizing the current drain for the bias circuit. While this approach has good temperature tracking characteristics, the output impedance of the bias circuit is much higher than optimal for non-constant envelope modulation schemes. The impedance of the temperature compensating circuit is further increased with the resistors 312, 314, and 315 while the output impedance is increased by any or all the resistors 304, 312, 317, and 318. The output impedance, disregarding the above listed resistors, is typically an order of magnitude (or ten higher) than that of the above current mirror with current boost. This is due to reducing the reference transistor to a diode by shorting out the collector/base junction. Also this form can also have a higher sensitivity to the output impedance changing with RF power resulting in a more pronounced gain expansion leading to possibly more distortion.